If a Tree Falls: Human Impacts on Forest and Park Trees

Forests are natural, wild places. Trees burn, blow down, mature, and regenerate on their own.

At the same time, forests have fingerprints of Homo sapiens all over them. If you know how to look, a stroll through Frick Park’s shady paths can highlight just how human actions have molded one very visible part of the park forests: the trees.

eastern hemlock magnified

An eastern hemlock attacked by hemlock woolly adelgid.

Dude, where’s my hemlock?

You could scour Frick Park and never come across white pine (Pinus strobus) or our state tree, the eastern hemlock (Tsuga canadensis). Until the 19th century both species were common throughout the state, but as the region industrialized, lumber companies prized these trees for their uses in tanning and construction and they disappeared from much of their former ranges. The result? The number of places in Pennsylvania where massive old-growth stands of white pine persist can be counted on your two hands. Afterwards, when forests began to regenerate, conditions did not always favor the return of these former giants.

If you do see a white pine or eastern hemlock in Frick (and there are a few of each), it was likely planted relatively recently by Parks Conservancy staff and volunteers. Much like logging these species, reforesting requires human labor.

A century later: Cherries

Though white pine and eastern hemlock have fared poorly in our forests over the last 200 years, the forests that regenerated after logging actually benefited other trees. Black cherry (Prunus serotina) is one of these. North of Pittsburgh, in the Allegheny Plateau, it is estimated that black cherry trees made up less than 1% of the pre-logging forest. After extensive clear-cutting in the late 19th century, however, black cherry became a common part of the new forest that regrew there thanks to preference for sunny conditions and fast growth.

The same is true in Frick Park. Easily identifiable by their dark bark that looks like burnt potato chips or corn flakes, black cherries are common these days. You can find a large number of them where there once was a country club (now in the area where Riverview and Bench trails run). After the club’s annexation to Frick in the 1920s, forests regrew on these lands with black cherries. Some of these trees are actually now dying, as the species’ mortality typically increases after 80-100 years.

Tagging Tree 2

Phil Gruszka, Parks Management and Maintenance Director, tagging a dead ash tree.

Invasive species, world travelers

Dead trees can also illustrate how humans have shaped our forests. Since 2007 when it was first detected in Pennsylvania, emerald ash borer (EAB), a tiny green invasive insect, has left a path of destruction across Allegheny County, killing nearly all area ash trees (Fraxinus var.) in just a few years. The evidence is all over Frick Park, with standing and fallen dead ash trees exhibiting the tell-tale scars where EAB larva chewed through the trees’ energy-rich cambium, girdling them.

Globalization not only redistributes products, money, and people around the world, but also non-native plants, animals, and fungi, sometimes in ways that reshape our parks. EAB, for example, likely arrived in the U.S. in a shipping pallet from Asia. This pest, however, is not the first invasive species to change our forests. In the early 20th century, chestnut blight, a fungus accidentally introduced from Asia, killed virtually all American chestnut trees, a species then common throughout the eastern U.S.

Trees on the move (and we’re not talking Ents)

Frick Park’s forests will keep changing as a result of human influence. In addition to the risk of future invasive species, anthropogenic (human-induced) climate change promises big shifts for park trees. Just walk Rollercoaster Trail on the hills between Fern Hollow and Falls Ravine and look out on the sea of young sugar maples (Acer saccharum) that dominate the forest understory.

According to the U.S. Forest Service’s Climate Change Atlas, sugar maples will likely become significantly less important across Pennsylvania as the climate warms and stresses this species, eliminating it from the southerly parts of its range. More heat-tolerant trees may ultimately replace sugar maples in Frick Park and elsewhere in the state.

Learning from the past

Recognizing human fingerprints on our forests gives us opportunity to learn from the mistakes and successes of past generations. How can we leave fingerprints that will improve forest health? Parks are planned spaces, cared for by the people that use them. The team at the Pittsburgh Parks Conservancy is constantly on the watch for invasives like the Asian longhorned beetle and oak wilt and replanting diverse, resilient species of trees to create strong forests that will be around for generations to come.

Kevin C. Brown is an educator with the Parks Conservancy, and a researcher-writer of a National Park Service-funded history of the Devils Hole pupfish, an endangered desert fish that lives in Death Valley National Park. You can read more about his work here.

[Presentation]: The Bambi Plague in the Eastern Deciduous Forest of the U.S.

Walt CarsonFor wildflowers and saplings in Pennsylvania’s forests, Bambi may well be Public Enemy Number One.  The overpopulation of white-tailed deer contributes to a host of problems.  Deer can turn a native plant into an invasive one, stop the regeneration of a forest understory dead in its tracks, and yes…they can even encourage colonies of voracious rodents.  Who knew one friendly-looking woodland creature could wreak so much havoc? 

Today’s presentation from “Preserving Pittsburgh’s Trees: Action and Recovery” is courtesy of University of Pittsburgh professor Dr. Walter Carson, who works in the fields of Plant Community Ecology and Tropical and Temperate Forest Ecology.  He shares some fascinating information about how the deer imbalance in the ecosystem leads to the dominance of only a few plant species and keeps the cycle of canopy regeneration from functioning properly.

Check out Dr. Carson’s presentation below, or download the slides only here.

Threats to the Urban Forest

EAB emergence holes

Emerald ash borer emergence holes on a tree in Schenley Park.

Pittsburgh’s four regional parks have 372,873 trees, according to the City of Pittsburgh’s recently completed Natural Areas Study. These trees represent 53 different species and collectively represent hundreds of millions of dollars in economic value, not to mention their aesthetic and environmental benefits.

But lately we’ve been hearing a lot about threats to those trees–pests like emerald ash borer and Asian long-horned beetle, and diseases like Dutch elm and oak wilt.  The emerald ash borer is in our parks–though we haven’t seen the beetle itself, the D-shaped emergence holes are becoming more and more obvious.  And oak wilt has forced the clearing of several acres of trees in Frick, Highland, and Riverview Parks to prevent further spread of the disease.

Oak wilt before and after

Before and after photos show the area of Frick Park where trees were removed due to oak wilt.

To add to the problem, many of the park trees are environmentally invasive, making them threats in and of themselves to the biodiversity of our forests–trees like Norway maple, tree of heaven, Siberian elm, and princess tree. It’s almost shocking how many of our park trees are invasive. In Schenley Park, it’s 32%; in Frick, it’s 35%; in Riverview, it’s 46%; and in Highland Park, a whopping 70% of the trees are invasive. Highland Park is basically a Norway maple forest, which is why it was so heartbreaking to see the stand of oaks removed recently due to oak wilt.  In other parks, we’ve been working to slowly phase out Norway maple and replant other natives, but in Highland Park many of the hillsides are entirely composed of these invasive trees.  Removing them all at once could cause serious erosion problems, so we have to wait until some of the smaller native trees grow large enough to keep the hillsides stable.

These are some of the many reasons that the Parks Conservancy is taking an active role in developing a tree action plan for our parks.  We’ve been talking to experts in government and academia to enlist their aid, as well as local groups like the Pittsburgh Shade Tree Commission and Tree Pittsburgh.  Our discussions have ranged from big dreams to practical solutions.  One idea being considered is to identify specimen trees–those that are particularly strong, beautiful, characteristic of their species, or definitive to an area–and develop a plan to save them (treating them for diseases if necessary). We can’t remove or treat every tree in the parks, because the process would disrupt the surrounding ecosystem. But we could make a dent. The City of Pittsburgh’s work to treat oak trees in areas where oak wilt is nearby is a great example of interventions that are being made to save trees.


A great specimen tree: white oak on the edge of the Westinghouse Woods by the Bob O'Connor Golf Course.

Funding is, of course, always a concern.  Remember that any contribution you make to our Emergency Maintenance Fund helps us preserve trees and respond to threats like these.

You can help out at your home, too. Keep a watchful eye on your own trees, especially ashes and oaks. If you have a particularly magnificent specimen tree and suspect something might be wrong, consult a certified arborist to see whether it can be treated. Research about emerald ash borer is ongoing; in some areas of the country where the infestation was at its peak several years ago, ash trees are beginning to regenerate. We won’t know for another 8-10 years whether the bugs will return and eradicate all the new growth, but some scientists say there is hope. So if you have trees you can protect, it’s worth a try.

On the other hand, if an infection in one of your trees is obvious, don’t wait to remove that tree. The emerald ash borer, for example, will kill a tree within three years, but the pests will remain in the tree until it is completely dead. If you remove the tree while part of it is still alive, you won’t be harboring the pests and you’ll reduce the population. When you do remove diseased trees from your yard, make sure the brush is chipped or burned and the logs are debarked before being used for firewood. The bugs and the fungus will remain below the bark and can continue to spread.

For more information on these threats, check out this great resource from the University of Maryland Extension. You’ll find more information about everything from managing gypsy moth to diagnosing what’s wrong with your trees.

Stay tuned for a lot more information–trees will be a primary focus of ours in 2011, with a public symposium planned for February where you can learn more about how to help.